Uses of Phosphoric Acid: from fertiliser to anti-nausea medication

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Phosphorous is vital for plant growth, so farmers use phosphoric acid-based fertilisers to supplement their crops

In simple terms, you can sayphosphoric acid is a weak inorganic acid with the formula H3PO4. However, it’s not as simple as it may seem … and phosphoric acid needs to be seen as a family and not an individual compound.

In this family, the simplest “member” is referred to as orthophosphoric acid and it represents a single phosphoric acid unit. When two or more of these orthophosphoric acid units “bump” into each other, they have a tendency to bind together, forming pyrophosphoric acid (2 units, H4P2O7), tripolyphosphoric acid (3 units, H5P3O10), tetrapolyphosphoric acid (4 units, H6P4O13) and so on. Collectively these are known as polyphosphoric acids. 

What is phosphoric acid?

At room temperature, pure phosphoric acid is a non-toxic, colourless, odourless, hygroscopic crystal. Commercial products are typically sold as a viscous solution in water, with varying concentrations according to use.

• Molecular formula: H3PO4 (orthophosphoric acid)
• Odour: odourless
• Colour: colourless
• Taste: Acid taste
• Boiling point: 407 degree Celsius
• Melting point: 42.4 degree Celsius
• Corrosivity: corrosive to ferrous metals, in particular highly concentrated solutions (85%)
• pH=1.5 in solution (0.1N)
• Solubility: soluble in water, alcohol and a 3:1 ether:alcohol mixture
• Forms 3 series of salts: primary, dibasic and tribasic phosphates
• Chelating agent
• Reacts readily with metals for form hydrogen gas
• Forms toxic oxides on combustion

Mode of preparation

In industrial settings, phosphoric acid can be produced via two separate routes, known as the wet process and the thermal process. Although the wet process is more predominant, the thermal process tends to be the preferred option for uses in the food industry.

• Wet process: This method involves treating fluoroapatite (known as phosphate rock, 3Ca3(PO4)2.CaF2) with concentrated sulphuric acid (H2SO4). The initial solution only contains about 30% phosphoric acid, but can be concentrated after water evaporation. In addition, by-product calcium sulphate (CaSO4) and other impurities can also be filtered and removed.
  Ca3(PO4)2 + 3H2SO4 -> 2H3PO4 + 3CaSO4
• Thermal process: This method generates a much purer phosphoric acid solution, as the starting point is pure elemental phosphorus, thus avoiding all impurities present in apatite. In this process, phosphorus is “burnt” to phosphorus pentoxide (P2O5) at high temperatures (1500-2500 degree Celsius) and then hydrated to phosphoric acid. Further purification steps may be needed for food or medical uses, to remove all traces of impurities.
  P4 + 5O2 -> 2P2O5
  P2O5 + 3H2O -> 2H3PO4

Phosphoric acid as fertiliser

For plants, phosphorus is an essential major nutrient. This means its functions cannot be performed by any other nutrient, and therefore an adequate supply is vital for development and growth. Farmers know this only too well, and commonly use phosphoric acid-based fertilisers to supplement their crops. In fact, the vast majority of the worldwide production of this compound is used in the manufacture of these agricultural products.

Typically the cheaper wet process is used, and several grades can be obtained, with varying concentrations of phosphoric acid. Products range from 0-20-0 (reflecting N-P-K content) to 0-79-0, with superphosphoric acid, depending on the “clean-up” steps following synthesis. Fertilisers are usually a mixture of ortho and polyphosphate, which are less corrosive than pure orthophosphate and easier to store at sub-freezing temperatures.

Phosphoric acid as a food additive

The second most common application is in the food industry, using phosphoric acid produced following the more expensive thermal process. Next time you go for a bottle of fizzy drink, check the ingredient list and most likely you’ll find E338. Typically, this additive is used to give various foods and drinks a sour and tangy taste. The list of examples is long, including from carbonated beverages and beer to chocolate and processed meat.

Niche applications

In addition to the two most common uses referred to above, phosphoric acid is used in a variety of niche applications, including:

• Rust removal: This compound can be simply applied directly on rusted iron or steel, where it can convert rust (iron(III) oxide, Fe2O3) to ferric phosphate (FePO4). Multiple applications may be needed.
  2 H3PO4 + Fe2O3 → 2 FePO4 + 3 H2O
• Protection against rust: Phosphoric acid can also provide some protection against the development of rust via a phosphate conversion coating. The anti-rust properties rely on a thin coating with a dilute solution of phosphoric acid. Reacting with the surface to protect, this coating forms a thin layer of insoluble crystals, impenetrable to rust attacks.
• Dentistry: Used by dentists as an etching solution. The phosphoric acid is used to “scratch” the smooth surface of the teeth, providing a grip for dental appliances or fillings. For the same reason, it can also be used as a plaque remover and teeth whitener.
• Anti-nausea medication: This compound is a common ingredient in several over the counter anti-nausea medications
• Cleaning products: Phosphoric acid can be a strong cleaning agent, typically used in construction to clean mineral deposits and cement stains. It’s also an ingredient in many household cleaning products, used as a dispersing agent.
• Other applications include:
  • buffer agent in chemistry and biology
  • electrolyte in phosphoric acid fuel cells
  • electrolyte in oxyhydrogen generators
  • electrolyte in copper electropolishing
  • oxidising agent
  • catalyst in the synthesis of ethanol
  • wet etching agent in compound semiconductor processing
  • pH regulator in cosmetic products
  • pH regulator in hydroponics

If you want to find out more, don’t forget to check out our resources list for detailed information about phosphoric acid.

RESOURCES

Pubchem.ncbi.nlm.nih.gov, (2014).Phosphoric acid – PubChem. [online] Available at:https://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=1004&loc=ec_rcs[Accessed 20 Aug. 2014].

ChemSpider, (2014). Phosphoric acid. [online] Available at:http://www.chemspider.com/Chemical-Structure.979.html [Accessed 20 Aug. 2014].

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Alex Reis

Alex Reis is a freelance science writer, with a particular expertise in the field of biological sciences. She has several years experience in scientific writing and research, with various scientific manuscripts published in high impact factor journals, including Nature Cell Biology, as well as articles promoted in more mainstream publications.